ULLMANN SYNTHESIS OF SIXDIMETHYLDINITROBIPHENYLS
May 5, 1956
bands a t 5.83(v.w.) and at 8.77(m-i) that are not shown by the diphenyline 111. Both I1 and I11 give bands a t 7.42 (v.w.), 9.92(w-m), 1 1 . 7 8 W and 11.95(m-i) where none exist in the spectrum of IV. Both 111 and IV show bands at 8.84(m) and a t 10.19-10.22(w-m) that I1 does not possess
.
[CONTRIBUTION FROM
The benzidine I1 possesses unique bands a t 7.64(w) and 7.74(i); the diphenyline I11 shows unique bands a t 7.66(m), 13.28(m) and 13.52(m); and the 2,2’-diaminobiphenyl IV has unique bands a t 7.68(i), 11,86(i, S) and 13.44(m).
PITTSBURGH, PENNSYLVANIA
DEPARTMENT O F CHEMISTRY, CARNEGIE INSTITUTE
THE
1997
O F TECHNOLOGY]
Ullmann Synthesis of Six Dimethyldinitrobiphenyls and their Reduction to the Corresponding Diaminodimethylbiphenylsl BY ROBERTB. CARLINAND GEORGEE. FOLTZ* RECEIVED SEPTEMBER 30, 1955 Six dimethyldinitrobiphenyls (I-VI) were prepared by simple and mixed Ullmann reactions. Catalytic reduction of these d o r d e d six corresponding diaminodimethylbiphenyls (VII-XII), all of which were converted to diacetyl and bissalicylal derivatives. Comparison of the diamines and of their derivatives with three diaminodimethylbiphenyls and their derivatives obtained by catalytic reductive debromination of the rearrangement products of 3,3‘-dibromo-5,5’-dimethylhydrazobenzene showed VII, I X and XI1 t o be the structures of the compounds derived from the rearrangement and thereby proved the structures of the rearrangement products themselves. Some aspects of the behavior of halonitrotoluenes in mixed Ullmann reactions is reminiscent of the behavior of monomer mixtures in copolymerization. This analogy may prove useful in the development of a satisfactory mechanism for the Ullmann reaction.
The rearrangement of 3,3‘-dibromo-5,5’-dimeth-comparison of their debrominated derivatives with ylhydrazobenzenea conceivably could lead to six synthetic specimens, the preparations of VIIdifferent diaminodibromodimethylbiphenyls. Cata- XI1 were carried out. This report is concerned lytic reductive debromination of these rearrange- with these preparations, all of which were achieved ment products could in turn yield six diaminodi- by Ullmann coupling of appropriate halonitrotolumethylbiphenyls (VII-XII). In order that all of enes and subsequent reduction of the resulting the rearrangement products could be identified by dimethyldinitrobiphenyls. The known compounds I, IV and V were obtained CHa CHa CHI CH3 in 25, 46 and 79% yields, respectively, by the symmetrical Ullmann coupling of 2-iodo-5-nitrotoluene, R - d - b - R R 4 b 4-iodo-3-nitrotoluene and 2-iodo-3-nitrotoluene respectively. Each of the three unsymmetrical diR methyldinitrobiphenyls 11, I11 and VI was preI. R = KO, 11. R = NO. VII; R = N H ~ VIII: R = NHZ pared by a mixed Ullmann reaction. Only one of VIIa, R = NHAc VIIIa, R = KHAc these (11) was obtained in crystalline form; I11 and VIIb, R NC;HIO VIIIb, R = NC7H;O VI were isolated as oils which probably were not CH3 R entirely free of isomers. The synthesis of I11 afI I fords a typical example of the behavior encountered in all three mixed Ullmann couplings. -4 35% yield of I11 was realized by treating equal weights 111, R = NO2 IX, R = NH2 of 4-bromo-3-nitrotoluene (XIII) and 2-iodo-5-niIXa, R = NHAc trotoluene (XV) with copper a t elevated temperaIXb, R = NC7H7O tures. A 4% yield of IV, from the symmetrical CH3 CHI coupling product I (from XV) was detected. When CHa&-&-cH3
IV, X, Xa, Xb,
d-b I
_ .
R R R R
= = =
I
R R V, R = NO2 X I , R = XHz XIa, R = NHAc X I b , R = NC7H7O
NO2 NHS NHAc XC7H70
R
R
CH3 VI, R NO2 XII, R = NHZ XIIa, R = NHAc XIIb, R = NCjI1;O ( 1 ) Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Carnegie Institute of Technology. (2) Institute Graduate Fellow in Organic Chemistry, 1951-1952. (3) R . R. Carlin and G . E. Foltz, THISJ O V R N . ~ L . 78, 1992 (1956,).
X
CH3 XIII, X = Br XIV, x = I
I
NO2 XV
x
XVI, X = Br XVII, x = I
4-iodo-3-nitrotoluene (XIV) was used in place of its bromine analog XI11 in an attempted synthesis of 111, only I V and unchanged XV were obtained from the mixture. The unsymmetrical dimethyldinitrobiphenyl I1 was prepared in a similar way in 16% yield from XV and 2-bromo-3-nitrotoluene (XVI); I1 was accompanied in the mixture by a 12% yield of V, from the self-coupling of XVI, but again no self-coupling product of XV was detected. Finally, a 32% yield of VI was obtained by coupling
ROBERT B. CARLIN AND GEORGE E. FOLTZ
199s
XIV with 2-iodo-3-nitrotoluene (XVII) ; this time both of the symmetrical biphenyls, IV from XIV and V from XVII, were also isolated in 13 and 16% yields, respectively. The formation of symmetrical IV and V, but not of I, as by-products of the mixed Ullmann reactions is worth noting. The precursors of IV ( X I I I and XIV) and of V (XVI and XVII) all have the ortho orientation of nitro and halogen, and I V and V were isolated whenever their precursors were used in a mixed Ullmann reaction. On the other hand, no I was obtained from either mixed Ullmann coupling in which XV, with its para orientation of nitro and halogen, was employed. Although the possibility of the presence of some I among the products cannot be excluded, its properties suggest that if present i t should be isolated more readily than either IV or V. Together with the observations that the simple coupling of XV to I occurs with difficulty and in poor yield and that XV was recovered unchanged from one attempted mixed Ullmann reaction, the foregoing experiences are consistent with the belief that no more than traces of I were formed as byproducts of the two mixed Ullmann reactions in which XV was a component. It therefore appears that XV is considerably more active in Ullmann reactions with different aryl halides than i t is in coupling with other molecules of its own kind. For example, toward XIII, XV appears about eight times more reactive than other XI11 molecules; yet XV is far less reactive toward molecules of its own kind than toward XI11 molecules.
1 ++ xv + + +xv+o Xvi + +
XI11
XI11
XI11
I11 (35%) IV (4.5%)
I11 (35%)
The mixed coupling of XV and XVI constitutes an entirely similar, if less striking, example. Such behavior of two components in a mixed Ullmann reaction bears some formal resemblance to certain copolymerization phenomena, in which monomer A may polymerize much more rapidly with monomer B than the latter does with itself in the monomer mixture, and yet A may be very sluggish in selfpolymerization. Inasmuch as copolymerization behavior of this type is now quite well rationalized, the analogy may prove useful in developing a satisfactory mechanism for the Ullmann reaction. Each of the dimethyldinitrobiphenyls I-VI was converted to the corresponding diaminodimethylbiphenyl VII-XI1 by hydrogenation a t room temperature over Raney nickel either in ethanol or in benzene solution. All but one (IX) of the six diamines was a crystalline solid, and the three symmetrical ones (VII, X, XI) had already been reported in the literature. An unstable crystalline hydrate apparently formed when crystallization of IX was attempted, for the solid that separated from solution changed to an oil when i t was dried or subjected to the action of anhydrous solvents in efforts to recrystallize it. One of the products of rearrangement of 3,3'-dibromo-5.5'-dimethylhydrazobenzene, on catalytic reductive debromination. yielded a substance that exhibited the same behavior. All six of the diaminodimethylbiphenyls were
Vol. 78
characterized as their diacetyl derivatives (VIIaXIIa) and as their bis-salicylal derivatives (VIIbXIIb). The three unsymmetrical diacetyl derivatives (VIIa, IXa, XIIa) and all but one (VIIb) of the bis-salicylal derivatives are reported here for the first time. Comparison by melting point and mixed melting point determinations of the diamines VII-XII, where these were crystalline solids, and of their crystalline derivatives with the three diamines and their derivatives obtained by catalytic reductive debromination of the rearrangement products of 3,3'-dibromo-5,5'-dimethylhydra~obenzene~ showed that VII, I X and XI1 were identical with the compounds derived from the rearrangement. The bissalicylal derivative X I I b derived from the rearrangement melted a t 171-172°, while a sample originating from the Ullmann reaction melted a t 208-210'; however, these were shown to be dimorphs and not different compounds when a mixture of the two a t first melted at 17G-210' and then, after being held in molten condition a t 220' for a few minutes and cooled until i t solidified, melted again a t 208-210'. The proofs of structure for the rearrangement products was therefore complete.
Experimental* 2-Iodo-3-nitrotoluene (XVII).-A solution of 50.0 g. of 2-amino-3-nitrotoluene6 in 400 ml. of hot glacial acetic acid was cooled to 15" and treated with an ice-cold solution prepared by adding 33 g. of sodium nitrite to 185 ml. of nearly frozen concentrated sulfuric acid, heating the mixture to 75' and then cooling again. The mixture was kept below 25" during the addition; then it was stirred and diluted to 1500 ml. with ice and water. The solution was stirred and treated successively with 33 g. of urea, with a solution of 79 g. of potassium iodide in 400 ml. of water, and then with sodium bisulfite until there was a distinct odor of sulfur dioxide. The crude solid product was collected by filtration and recrystallized from aqueous ethanol, from which it formed 59.0 g. (68.2%) of pale yellow cubes, m.p. 66-88', The m.p. 67-68' has been reported.' 4-Iodo-3-nitrotoluene (Xnr) and 2-iodo-5-nitrotoluene ( X V ) were prepared in 70 and 72% yields, respectively, from the corresponding aminonitrotoluenes (both from Eastman Kodak Co.) by the procedure described in the preceding paragraph: XIV, light yellow needles from petroleum ether (b.p. 30-60°), m.p. 55-56', reported m.p. 55"; XV, small white needles from ethanol, m.p. 103-105", reported m.p. 103-104°.8 4-Bromo-3-nitrotoluene (XIII).-A solution prepared by stirring 50.0 g. of 4-amino-3-nitrotoluene into 200 ml. of 1: 1 (by volume) sulfuric acid was cooled t o 15' and treated with a nitrosylsulfuric acid solution obtained just as described in the procedure for the preparation of XVI, above. A suspension of cuprous bromide was prepared by mixing solutions of 60 g. of cupric sulfate in 200 ml. of water, 30 g . of potassium bromide in 70 ml. of water and 110 ml. of 48% hydrobromic acid and then saturating the mixed solution with sulfur dioxide. After the cuprous bromide had been added to the diazonium solution, the mixture was subjected to steam distillation, and the distillate was extracted with ether. The ether solution was washed with sodium hydroxide and with water and was then dried over calcium chloride. Distillation of the ether left 31.5 g. (44.0%) ?f a residual oil, b.p. 97-103' (1 mm.). The distilled oil solidified and the solid was recrystallized from ethanol, from which it formed yellow prisms, m.p. 32-33", reported m.p. 3 3 O . O (4) Melting points are corrected. 15) J . H. Hadfield and J. Kenner, Proc. Chcm. SOC.London, 28, 253 (1914). (6) H. L. Wheeler and L. M. Liddle, A m . Chcm. J . , 42, 441 (1909)
(7) C. Willgerodt and M. Simoais. Bcr., SB, 269 (19061. ( 8 ) F. Reverdin and K . Kacer, ibid., SO, 3000 (1897). (9) A. P. Hollemann, Rcc. Iran. chim., 84, 285 (1914).
ULLMANN SYNTHESIS OF SIX DIMETHYLDINITROBIPHENYLS1999
May 5, 1956
2-Bromo3-nitrotoluene (XVI) .-A diazonium solution was prepared from 25.0 g. of 2-amino-3-nitrotoluene by means of the same procedure used in the preparation of XVII, above. The diazonium solution was treated with 12 g. of urea and then with a cuprous bromide suspension obtained by mixing solutions of 40 g. of cupric sulfate and 20 g. of potassium bromide in 120 ml. of water with 8.2 g. of sodium metabisulfite and 5.4 g. of sodium hydroxide in :6 ml. of water, the temperature being maintained a t 50 , and then adding 80 ml. of 48% hydrobromic acid. The mixture was heated to 85", allowed t o stand about 12 hours and then extracted with ether. The ether extract was washed with water and with 10% aqueous sodium hydroxide and then dried over anhydrous potassium carbonate. Distillation of the ether left an oily residue, from which 25.6 g. (72%) of XVI distilled, b.p. 78-84' (1 mm.). The distillate formed pale yellow plates, m.p. 38-40", from ethanol. The reported m.p. is 41-42".1° 4,6 '-Dimethyl-? ,2'-dinitrobiphenyl (VI) .-An intimate mixture of 19.0 g. of 2-iodo-3-nitrotoluene (XVII), 17.0 g. of 4-iodo-3-nitrotoluene (XIV) and 36.0 g. of copper bronze powder (0. Hommel Co., Pittsburgh, Pa., Extra-fine Copper No. 5743-hereafter t o be referred t o simply as "copper powder") was heated for two hours a t 110 O and then gradually heated to 200 The cooled mixture was extracted with benzene; removal of the benzene from the extract left a dark residual oil which on vacuum distillation yielded 15.2 g. of a n orange oil, b.p. 164-166' (1 mm.). A solution of the oil in ethanol when cooled deposited 2.4 g. (12.9%) of 4,4'-dimethyld,2'-dinitrobiphenyl (IV), m.p. 142-143" after recrystallization from ethanol. The ethanol was removed from the mother liquor from IV, and the residual oil was dissolved in ether-petroleum ether (b.p. 30-60"). When cooled this solution deposited 5.8 g. of material from which 2.9 g. (15.670) of 6,6'-dimethyl-Z,2'-dinitrobiphenyI (V), m.p. 110-111 , was obtained by recrystallization from the same solvent mixture. Removal of solvents from the filtrate from V left 'a liquid which gave 6.0 g. (32%) of an oil, b.p. 166-168' (1 mm.), after vacuum distillation. Although this oil must have consisted chiefly of VI, it probably also contained small amounts of isomers. Anal. Calcd. for Cl~HlnN~04:C, 61.76; H , 4.44. Found: C, 62.36; H , 4.51. 2,4'-Dimethyl-2,4'-dinitrobiphenyl (111).-A mixture of 20.0 g. of 2-iodo-5-nitrotoluene (XV), 20.0 g. of 3-nitro-4bromotoluene ( X I I I ) and 40.0 g. of copper powder was heated t o 145", when the temperature suddenly surged t o 275'. The mixture was allowed t o cool t o about 200' and then heated t o when the temperature again surged, this time t o 320 The cooled mixture was extracted with benzene; distillation of the benzene left a dark brown oil from which 10.0 g. of a heavy orange oil, b.p. 215-217' ( 3 mm.) was obtained by vacuum distillation. A solution of this oil in ether-petroleum ether (b.p. 30-60') deposited 1.O g. (4.4%) of 4,4'-dimethylD,2'-dinitrobiphenyl (IV), m.p. 141-142 ' after recrystallization from ethanol, when it was cooled in Dry Ice. Removal of the solvents from the filtrates from IV left a liquid from which 8.0 g. (35.2%) of a yellow oil, b.p. 204-206' (1 mm.), was obtained by vacuum distillation. Although the transformation products IX, I X a and I X b from this oil showed that it must have consisted principally of 111, the presence in it of one or more isomers of I11 is highly probable. Anal. Calcd. for C14&N204: C, 61.76; H, 4.44. Found: C, 62.36; H, 4.78. 2,2'-Dimethy14,6'-dmitrobiphenyl (11).-A mixture of 18.3 g. of 2-iodo-5-nitrotoluene (XV), 15.0 g. of 2-bromo-3nitrotoluene (XVI) and 30.0 g. of copper powder was heated until the luster of the copper powder disappeared and the powder became dark in color. Treatment of the resulting mixture exactly like that described in the preceding section gave first 2.2 g. (11.6%) of 6,6'-dimethyl-2,2'-dinitrobiphenyl (V), m.p. 109-111 'after two recrystallizations from methanol, and then 3.0 g. (15.8%) of 2,2'-dimethyl-4,6'dinitrobiphenyl (111) as an oil after distillation. A solution of this oil in ether-petroleum ether (b.p. 30-60') was kept at 5' for a week, during which time small amounts of petroleum ether were added. This treatment caused the separation of short yellow needles, which after roecrystallization from aqueous methanol had the m.p. 84-85
'.
yo,
.
.
(10) C. S. Gibson and J. D. A. Johnson, J . Cham. SOL, 1229 (1929).
Anal. Calcd. for C1,HuN20r: C, 61.76; H, 4.44. Found: C, 61.37; H , 4.57. 2,2 '-Dimethyl-4,4 '-dinitrobiphenyl (I).-A mixture of 10.0 g. of 2-iodo-5-nitrotoluene (XV) and 1060 g. of copper powder was heated for a half-hour a t 280 The cooled mixture was extracted with benzene. Distillation of the benzene from the extract left 5.3 g. of tar which contained some crystalline material. Much of the dark colored contamination was removed by dissolving the residue from the benzene solution in acetone and then precipitating some of the tar by adding boiling petroleum ether (b.p. 65-110'). The solvents were removed from the resulting solution, and unchanged XV was removed from the residue by sublimation a t 100" (10 mm.). When the residue from the sublimation was washed with a smatl amount of cold ether, the tar dissolved and a yellow powder remained which yielded 1.27 g. (25%) of 2,2'-dimethyI-4,4'-dinitrobiphenyl ( I ) by sublimation at 150-160' (1 mm.). Ethanol recrystallization afforded small yellow cubes, m.p. 170°, reported m.p. 170°.'1 The substantial improvement in yield over previous efforts to carry out this same reaction12 is attributed to the higher temperatures employed in the present work. 2,2 '-Dimethy14,6'-dinitrobiphenyl (V) and 4,4'-Dimethyl2,2'-dinitrobiphenyl (IV).-A mixture of the appropriate iodonitrotoluene with an equal weight of copper powder (and, in the case of the preparation of IV, twice its weight of clean sand) was heated t o 180' for the preparation of IV and 200" for V. The mixture was extracted with benzene, the benzene was removed from the extract by distillation, and
.
TABLE I PROPERTIES AND BIPHENYLS AND
Com-
ANALYTICAL DATAFOR DIAMINODIMETHYLFOR THEIRDIACETYLAND THEIRBISSALICYLAL DERIVATIVES
pound
M.p., "C.
VI1 VI11 IX X XI XI1
105-106d 88-89 Oil* 125-126' 135-136' 106-107
VIIa VIIIa IXa Xa XIa XIIa
284-285& 181-182 183-184 192-1 93' 206-207' 174-175
Carbon, %
Calcd.
Found
Hydrogen, % Calcd. Found
Diamines"
...
...
..
..
79.21 79.21
79.62 78.64
7.60 7.60
7.81 7.28
...
...
... ...
79.21
79.41
.. ..
7.60
Diacetyl deriv.'
..
..
7.71
...
...
..
..
72.95 72.95
72.67 73.09
6.80 6.80
6.99 6.84
..,
...
..
.. ..
6.80
6.36
...
72.95
...
73.17
..
Bis-salicylal deriv.'
..
VIIb 200-206 ... ... .. VIIIb 79.97 80.09 5.75 5.94 134-136 IXb 5.71 170-172 79.97 79.70 5.75 6.09 Xb 79.97 79.86 5.75 168-170 XIb 5.78 5.75 234-235 79.97 80.15 XIIb 5.75 5.75 208-210' 79.97 80.17 o Formula: ClCHl~N~.* Formula: CI~H,ON~OZ.Formula: C&H*,N~OZ. Reported m.p. 106-107", P. Jacobson and 0. Fabian, Ber., 28, 2553 (1895). 'Forms hydrochloride, m.p. 260" dec. Anal. Calcd. for CirH1BN<: C, 58.95; H, 6.36. Found: C, 59.35; H, 6.25. Reported m.p. l2Oo.l4 0 Reported for DL-form, m.p. 136"; J. Kenner and W. V. Stubbings, J. Chem. Soc., 119, 600 (1921). As reported." Reported m.p. 189O.l' Reported for DLform, m.p. 205"; cf. ref. under g. kReported m.p. 200201 O . 1 1 1 Dimorphic form, m.p. 171-172" obtained by catalytic reductive debromination of one of rearrangement products of 3,3'-dibromo- 5,5' -dimethylhydrazobenzene. See last paragraph of Introductory section. (11) F. Ullmann and L. Frentzel, Bcr., 88, 729 (1905). (12) R . B. Carlin and R. C. Odloso, THIS JOURNAL, 76, 2345 (1854).
2000
B. S.FRIEDMAN AND FRED L. MORRITZ
the residue was crystallized from ethanol. In this way, 2iodo-3-nitrotoluene (XVII) gave a 79% yield of V, yellow needles, m .p. 109-1 10 O ; and 4-iodo-3-nit:otoluene (XIV) afforded IV, yellow needles, m.p. 141-142 , in 46% yield. The reported m.p.'s are l l O o l * for DL-Vand 142-143OI4 for IV. Hydrogenation of Dimethyldinitrobiphenyls to Diaminodimethylbipheny1s.-The dimethyldinitrobiphenyls were treated with hydrogen a t 45-50 p.s.i. a t room temperature in the presence of Raney nickel for 4 hours (except for IV (1 hour) and V ( 2 hours)). Solvents were benzene for the preparations of I X and X and absolute ethanol for the other four. The hydrochlorides of I X and X were precipitated from the benzene solutions by treating the benzene solutions of the bases with dry hydrogen chloride, the hydrochlorides were dissolved in water, and the bases were precipitated by treating the solutions with aqueous sodium hydroxide. The diamine I X was an oil which was extracted into ether. Evaporation of the ether left an oil which gave a solid, m.p. 56-64', from aqueous ethanol. This solid, probably an unstable hydrate, reverted to an oil when dried or when attempts were made to recrystallize it from non-aqueous solvents. The other four diamines (VII, VIII, X I and X I I ) were (13) W. Dethloff and H. Mix, Chem. Bcr., 82. 534 (1949). (14) St. van Niementowski, i b i d . , 84, 3325 (1901).
Vol. 78
precipitated from their solutions in ethanol by concentrating the solution and then diluting with water. All of the diamines were recrystallized from aqueous ethanol except for I X (see above) and VIII, which was recrystallized from aqueous methanol. Properties of and analytical data for the six bases are recorded in Table I. Diacetyl Derivatives.-All of the diacetyl derivatives except VIIa were prepared by bubbling ketene into ether solutions of the corresponding diamines. The diacetyl derivatives, which precipitated from the solutions as they formed, were collected by filtration. The diamine VI1 was converted to VIIa by means of acetic anhydride, in accordance with procedure A for the acetylation of diamines described in the preceding paper.8 All but one of the diacetyl derivatives were recrystallized from aqueous ethanol; VIIIa was recrystallized from water. Table I includes the properties and analytical data for VIIa-XIIa. Bis-salicylal Derivatives.-The diamines were heated with excess salicylaldehyde at 100' for 0.5-2 hours (VII, IX, X I I ) or a t 190' for 5 minutes (VIII, X , XI); then most of the excess salicylaldehyde was removed by distillation. Except for VIIb, all of the derivatives were recrystallized from ethanol; petroleum ether (b.p. 65-110") was the solvent for VIIb. Properties and analytical data for these compounds are reported in Table I .
PITTSBURGH 13, PENNSYLVANIA
[CONTRIBUTIONSFROM SINCLAIR RESEARCH LABORATORIES, INC.]
The Alkylation of Benzene with Isoamylenes and with f-Pentyl Chloride] BY B. S.FRIEDMAN AND F. L. MORRITZ~ RECEIVEDOCTOBER 13, 1955 As reported by others, the ratio of t-pentylbenzene ( I ) to 2-phenyl-3-methylbutane (11) obtained by the reaction of benzene with t-pentyl chloride is dependent on the type and activity of catalyst employed. This applies also t o the reaction of benzene with (A) 2-methyl-2-butene and (B) 3-methyl-1-butene. However, other factors of reaction severity also affect the course of the reaction. Thus, an increase in temperature, contact time or amount of catalyst decreases the ratio of I to I1 obtained with a given catalyst, The type of olefin used also affects the ratio, e.g., aluminum chloride produces pure J at -40" and pure I1 a t 21" from (B), but gives a mixture of I and I1 a t 21' with ( A ) . The mechanism of the reaction is discussed.
Boord2 and his co-workers reported on the specificity of catalyst action when benzene was alkylated with t-pentyl chloride. The strong FriedelPCrafts catalyst, aluminum chloride, gave a mixture of isomeric pentylbenzenes. On the other hand, ferric chloride or aluminum chloride dissolved in nitromethane, produced pure t-pentylbenzene. In the alkylation of benzene.with branched chain hexyl chloride, or branched hexenes, Schmerling and West3 demonstrated that the type of substitution was greatly dependent upon the catalyst. At 0" the aluminum chloride-catalyzed alkylation of benzene with l-chloro-3,3-dimethylbutaneor 2chloro-2,3-dimethylbutane resulted only in the formation of the secondary isomer, 2-phenyl-3,3-dimethylbutane, in the monoalkylated product. Aluminum bromide and zirconium chloride, the latter a t 85", gave very similar results. The other Friedel-Crafts catalysts investigated, e.g., aluminum chloride dissolved in nitromethane, catalyzed the reaction to give only the t-hexylbenzene, 2phenyl-2,3-dimethylbutane,in the monoalkylated product. Two mechanisms have been proposed3 for the (1) Presented at the 128th Meeting of the American Chemical Society, Minneapolis, Minn., September, 1955. (2) M. Inatome, K . W. Greenlee, J. M. Derfer and C. E. Board, THISJ O U R N A L , '74, 292 (1952). (3) L . Schmerling and J. P. West, ibid., 76, 1917 (1954).
seemingly anomalous formation of secondary alkylate. One mechanism assumes that the reaction occurs via concerted bimolecular nucleophilic dis) the benzene acting upon placements ( S N ~with a complex of the alkyl chloride and the catalyst. The other mechanism postulates initial tertiary alkylation and subsequent isomerization t o give secondary alkylate. There appears t o be a correlation between the isomerizing ability of the catalyst and the extent to which secondary alkylation occurs. It is the purpose of the present investigation to show that the extent to which secondary alkylate is formed is dependent upon the total severity of the reaction. Higher operating temperatures, longer reaction times and increased catalyst ratios appear to favor the formation of secondary alkylate. Effect of Temperature.-As shown in Table I, alkylation of benzene with 2-methyl-2-butene a t 21" in the presence of aluminum chloride resulted in a yield of pentylbenzenes. The composition of this product was 55% t-pentylbenzene (I) and 45y0 2-phenyl-3-methylbutane (11). The yield of pentylbenzenes was not much lower a t 0" and a t -40", but a t 0" the ratio of I to I1 was 75/25, and a t -40" only I was formed. When 3-methyl-lbutene was used as the alkylating agent, the effect
